It is permissible to use a phoropter or trial frame to determine best-corrected Snellen visual acuity. The left eye is occluded first. An approximate beginning refraction may be determined by retinoscopy, automated refraction, or a subjective refraction from a prior visit. The sphere is refined first. The cylinder is then refined, first the axis followed by the power. Finally, the sphere is rechecked. The right eye is then occluded, and the procedure is repeated for the left eye.